Getting TOP n Items in a Table/chart

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I have gone through the forum and have found many queries which pertain to the above topic (have searched on TopN and RankX). I have tried all of them and have not been able to come up with the correct result. 

I will try and put my problem in as simple terms as I can. So here goes.

I am trying to create some analytics for a restaurant. I want to show the Top 10 items which were sold (by quantity) withing a given date range (dates being select from a slicer - picking years or months)

So I created a measure for summing up my quantities

Msr_SaleQty = sum('sales'[quantity])

Next I created another measure to get the Rank

Msr_Rank_by_qty = RANKX(ALL('sales'[item]),[msr_saleqty])

So far so good.

Now I create a table visualization and add the item, quantity and the Rank in it. ( i have just done Top 3 in the example)

This works perfectly.

However If I add a month column into this table, I get the ranking for each month. So If I have selected 2 months, then I will get 6 rows.

But I just want the composite ranking within whatever periods I have selected.

I have looked at the following article

RANKX-apalooza

Tried to change my ranking measure according to this 

Created another measure as follows

Msr_Rank_by_qty1 = RANKX(ALLSELECTED('sales'),CALCULATE(sum('sales'[quantity])))

Now the results are worse..